1. Field of the Invention
The present invention relates to a satellite positioning apparatus, in a satellite positioning system such as the Global Positioning System (GPS), for determining the position of the satellite positioning apparatus based on signals received from satellites.
2. Description of the Related Art
GPS is a widely used satellite navigation system that includes a plurality of satellites and a satellite positioning apparatus that moves with a mobile object such as an automobile.
Positioning technology for GPS receivers select from satellites with receivable signals, a combination of satellites in which the optimal dilution of precision (DOP) may be determined depending on the geometric constellation of the satellites. A position by using signals received from the satellites may also be determined (e.g., Japanese Unexamined Patent Application Publication No. 5-297104).
Additionally, position determination and positioning error calculation for each of different combinations of satellites may be provided. Based on the determined position and positioning error, a range in which the error regions obtained for the combinations of the satellites mutually overlap presents the range in which the present position can exist (e.g., Japanese Unexamined Patent Application Publication No. 6-11560).
In addition to position determination with a GPS receiver, a velocity vector may also be calculated based on Doppler shifts of signals received from satellites (e.g., Japanese Unexamined Patent Application Publication No. 6-66916).
Positioning accuracy and error tendency vary depending on the combination of satellites used in position determination. Accordingly, a change in the combination of satellites used in position determination may cause a deterioration in positioning stability of a satellite positioning apparatus, so that large shifts in determined positions may occur. The occurrence of the large shifts in determined positions may cause a user to question the performance of the GPS system. Additionally, it is difficult to correct the position determined by the satellite positioning apparatus by using techniques such as map matching, which perform correction by collating the determined position with previously determined positions, loci represented by the positions' records, and a road map. Moreover, it often occurs that map matching further increases the shifts in positions.